Police are investigating the fatal shooting of six young revellers at a house party in the US city of Seattle.

They were killed, and two others wounded, when a fellow guest went on a gun rampage before finally killing himself when confronted by police.

They were at a "zombie party", where guests wear make-up to make them look dead, in the city's Capitol Hill area.

Police are baffled as to what sparked the incident, which took place in the early morning as the party wound down.

"We have absolutely no idea what the motive is," Seattle police chief Gil Kerlikowske said. "These appear to be almost execution-style shootings. He expended a large number of rounds inside that house."

People inside the house began running and screaming in terror

Seattle police chief Gil Kerlikowske

The victims were five males and two females, aged from their mid-teens to their 20s, police said.

"It was a zombie party. People had a lot of make-up on," Mr Kerlikowske said. "There was fake blood squirted, and people were made-up to look as if they were dead."

Weapons in truck

The gunman, in his 20s and believed to be a Seattle resident, is reported to have been invited to the house party after attending a "zombie rave" for about 1,000 people, also in Capitol Hill, earlier on Friday night.

This woman is awaiting news of her boyfriend, who was at the party

As the house party was winding down at 0700 (1500 GMT) on Saturday morning, the killer reportedly left the house and went to his truck, parked nearby to collect a pistol grip shotgun and handgun and to don bandoliers of spare shotgun shells.

As he walked the half block back to the house he paused to spray paint the word "Now" in three spots - twice on the pavement and once on the steps of a neighbour's home, police said.

Once back inside the house he started shooting.

"People inside the house began running and screaming in terror," Mr Kerlikowske said. "A pistol-grip shotgun is a weapon not designed for hunting purposes but for hunting people."

Hiding in bathroom

Having killed two people on the ground floor, he then went upstairs looking for more victims, police said.

He tried to get into a bathroom where two girls were hiding. When the locked door impeded him he fired through the door, but the girls were left unharmed.

The killer spray painted the word 'Now' three times

The gunfire was heard by neighbours and a police officer just a block away, who rushed to the scene.

When the police officer, Steve Leonard, arrived he found one person staggering out of the house with a gunshot wound.

He confronted another man with a shotgun, who turned it on himself after being told several times to put it down, Mr Kerlikowske said.

About 20 party guests were taken to Seattle police headquarters for counselling and questioning, police sergeant Deanna Nollette said.

"This is basically one of the most serious and significant crimes we have had in this city. The number of young people who witnessed what happened is disturbing."

Police said they did not know whether drugs or alcohol were a factor, though Mr Kerlikowske said marijuana and alcohol were found in the house.